This invention relates to accurate and automatic control of the molding time for rubber compounds and the automatic opening of rubber-molding presses when the cure is calculated to be complete.
Much time-temperature cure data for rubber compounds is known, and each manufacturer of rubber products usually has some of these data at his disposal. The usual way of operating rubber-molding presses is for the operator to load them manually and for the operator then to close the press. Closure of the press operates a timer which has been preset for a time at which cure should be completed in view of what is supposed to be the temperature of the mold. However, the mold temperature, even though it is thermostatically maintained, is not likely to be identical with this supposed temperature. The actual temperature of the mold may vary rather widely, and the correction of the temperature by the thermostat may take some time. For example, the amount of time that the press is open during the operator's loading of the press varies, and the longer the press is open, the cooler the mold is when it is closed and again starts heating. Thus, it may be many degrees below its nominal temperature when the mold is first closed, and it may take a substantial amount of time for the mold to reach this nominal temperature. The thermostats are usually actuated within a plus or minus 2% to cause the device to heat until it reaches the nominal temperature, but this is not sufficient to assure that the temperature has been maintained as an average during the entire molding operation--as a matter of fact, it rarely if ever has.
Because of these inaccuracies, the practice in the industry has been to calculate the cure time as the shortest time in which one can be absolutely certain that all parts will be cured with any reasonable amount of mold-opening time during unloading of the previous batch and reloading. This, of course, means that the rubber will tend to be overcured in almost every instance, because the worst cure time will not be so often met with. It also means that if there are times in which the mold is opened longer than was thought or in which the mold temperature for some other reason did not rise in time, that even the nominally worst time will not be so bad that some batches will be undercured.
This practice has had two serious economical effects: in the first place, many batches have to be discarded when after tests they are found either to be undercured or overcured beyond the tolerance limits. This has been a serious problem in many factories where synthetic rubber has been cured. In the second place, it means that the molds are kept occupied and are closed much longer than they need to be to obtain the best results. This means that fewer products can be molded per unit time and per hour of operator work. As a result, there has been substantially less production than would have been possible had the actual cure time been known and followed by the mold.
It is an object of the present invention to achieve more exact cure of the rubber material being molded so as to substantially reduce the number of defectively cured batches that have to be rejected. Another object is to reduce substantially the amount of time in which the presses are closed unnecessarily. By accurate and constant calculation and recalculation of the correct mold time under the temperatures actually present in the mold, the material can be cured accurately and can be relied upon to produce very few rejections, perhaps completely eliminating all rejections due to faulty mold cure. Furthermore, the mold and the operator can be much more efficiently employed.